The present invention relates to a method and a device for operating a laser diode, whereby the intensity of the laser beam produced by the diode is adjusted with an adjustment signal and directed along a path to stock, such as printer stock or photographic stock, that is to be illuminated. Under certain conditions unstable reflections from the stock, acting as a reflector, act on the diode with respect to time and/or space, thus interfering with the laser intensity.
Laser diodes are employed in many fields of communications. They are particularly practical because the intensity of the light they emit can be modulated. Since they are so small and easy to control electronically, laser diodes can be employed to transmit data very rapidly. This feature is of significant advantage not only for transmitting but also for reproducing information.
Laser diodes, however, do have one drawback in that they tend to shift spontaneously from one mode to another at specific ranges of intensity. This tendency results in the low-frequency intensity fluctuations called mode hopping. Mode hopping is particularly annoying when laser diodes are employed to switch statically or dynamically, especially for signal modulation.
Several ways to avoid mode hopping have already been suggested. The U.S. Pat. No. 4,817,098 for example proposes a system that involves various means of maintaining laser outputs constant. The temperature of the diode can be measured for instance and regulated to minimize temperature fluctuations and accordingly maintain its output as constant as possible.
The U.S. Pat. No. 5,283,793 suggests superposing a high-frequency signal over an imaging signal being applied to printer stock.
US Pat. Nos. 4,799,069 and 5,386,124, finally, suggest preventing mode hopping by briefly turning the laser diode off between every pair of pixel signals.
These approaches to the prevention of mode hopping are either technically rather complicated or lead to relatively unsatisfactory results. One object of the present invention is accordingly a method and device for preventing the sudden low-frequency disruptions in laser intensity that mode hopping causes. A more specific object is to stabilize the light emitted by a laser diode scanning a surface with respect to mode hopping when reflections from the surface interfere with the diode.
These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention, by producing intentional and controlled feedback of enough of the light emitted by the laser diode back to the diode's effective surface, via another path, to maintain the laser diode in a state of coherence collapse.